Heavy oil and bitumen account for more than double the resources of conventional oil in the world. Recovery of heavy oil and bitumen is a complex process requiring products and services built for specific conditions, because these fluids are extremely viscous at reservoir conditions (up to 1500000 cp). Heavy oil and bitumen viscosity decreases significantly with temperature increases and thermal recovery methods seems to be the most promising ones. Steam Assisted Gravity Drainage (SAGD) offers a number of advantages in comparison with other thermal recovery methods. Typical implementation of this method requires at least one pair of parallel horizontal wells drilled near the bottom of the reservoir one above the other. The upper well, “injector,” is used for steam injection, the lower well, “producer,” is used for production of the oil. SAGD provides greater production rates, better reservoir recoveries, and reduced water treating costs and dramatic reductions in Steam to Oil Ratio (SOR).
One of main problems of thermal recovery methods is the processes start-up. Due to high viscosity the cold oil is essentially immobile and therefore initial reservoir heating is required. This initial preheating stage is necessary to create a uniform thermo-hydraulic communication between the well pair, or create a heated zone around the well in the case of single well completion. During this start-up period, steam circulated in well(-s) to heat the reservoir and no (or little) oil production is assumed. This stage requires a lot of energy to be injected into the reservoir with the steam. Preheating stage strategy aims at minimizing the time in which well(-s) can be converted to the oil production operation regime as well as minimization of the amount of steam needed for circulation.
Thermal methods of heavy oil recovery are described in U.S. Pat. No. 4,085,803, published Apr. 25, 1978, U.S. Pat. No. 4,099,570 published Jul. 11, 1978 and in U.S. Pat. No. 4,116,275 published Sep. 26, 1978. Description of the SAGD process and its modifications is given in U.S. Pat. No. 4,344,485 published Aug. 17, 1982.
U.S. Pat. No. 6,988,549 published Jan. 24, 2006 discusses certain problems associated with typical SAGD projects. According to this patent the economics of such projects is significantly impacted by the cost associated with steam generation and SAGD does not typically employ the use of super-saturated steam because of the high cost of producing this steam with conventional hydrocarbon-fired tube boilers which results in using steam that is less efficient in transferring heat to the heavy oil reservoir.
The economics of SAGD may have been adversely affected by the duration of the preheating stage and the circulation steam rates at this stage. Commercial simulator numerical models were used to estimate SAGD preheating parameters (steam circulation rate and preheating stage duration). In particular: Vanegas Prada J. W., Cunha L. B., Alhanati F. J. S.: “Impact of Operational Parameters and Reservoir Variables During the Startup Phase of a SAGD Process,” SPE paper 97918; Vincent K. D., MacKinnon C. J., Palmgren C. T. S.: “Developing SAGD Operating Strategy using a Coupled Wellbore Thermal Reservoir Simulator,” SPE paper 86970; Shin H., Polikar M.: “Optimizing the SAGD Process in Three Major Canadian Oil-Sands Areas,” SPE paper 95754.
Nevertheless these models cannot be used for the fast estimation of the optimal preheating parameters for a wide range of reservoir properties and do not consider necessary changes of the well operating regimes at various time intervals of the preheating stage.
U.S. Pat. No. 5,215,146 published Jul. 1, 1993 describes one of the realizations of preheating process. Method given in this patent can reduce the duration of the preheating stage. In the described process steam is circulated in both horizontal wells with the constant considerable temperature gradient in-between them, which forces the heated fluids to move from upper to lower well. Certain amount of foam is injected in order to increase pressure gradient between the wells and hence the oil phase velocities. Increased oil rates will decrease preheating time but only after the time moment when thermo-hydraulic communication between well pair was achieved. This method is energy- and capital-expensive, as foam production requires additional resources and equipment. Associated oil/water/foam production control procedure is also too complex.